In this study was evaluated the relationship between self-reported smoking rate and expired air carbon monoxide in 208 smokers who had attended a behavioral program for smoking cessation. A close relationship between carbon monoxide levels and self-reports was found at the end of treatment and in all follow-ups (6 and 12 mo.), around 100% concordance. Thus, support was found for the use of an expired air carbon monoxide measure as a valid and easy way of corroborating self-report data when required.